1. Field of the Invention
The present invention relates to an antenna unit which is surface-mountable on a circuit board or the like, and more particularly, it relates to a surface-mountable antenna unit which is preferably used in a mobile communication device or the like, for example.
2. Description of the Background Art
An antenna unit must be excellent in characteristics such as the gain and return loss, while further miniaturization is required for an antenna unit which is applied to a mobile communication device or the like.
In general, (a) an inverted-F antenna unit, (b) a microstrip antenna unit and (c) a dielectric-loaded monopole antenna unit are known to be those which are suitably used in high frequency ranges.
An example of the inverted-F antenna unit (a) is described in "Small Antennas" by K. Fujimoto, A. Henderson, K. Hirasawa and J. R. James, Research Studies Press Ltd., England. With reference to FIG. 1, an exemplary inverted-F antenna unit 1 is now described. The inverted-F antenna 1 has a rectangular metal plate 2 which serves as a radiating part. An edge of the metal plate 2 is partially perpendicularly bent to form a ground terminal 3. Another edge of the metal plate 2 is also partially bent to form a feed terminal 4.
Due to the aforementioned structure, it is possible to mount the inverted-F antenna 1 on a printed circuit board by inserting the ground terminal 3 and the feed terminal 4 in through holes which are provided in the printed circuit board.
In the inverted-F antenna 1, however, it is difficult to reduce the metal plate 2 in size due to an insufficient gain. Further, the printed circuit board for receiving the antenna 1 must be provided with through holes for receiving the ground terminal 3 and the feed terminal 4. In other words, it is impossible to surface-mount the inverted-F antenna 1 on the printed circuit board.
An example of the microstrip antenna unit (b) is described in "Microstrip Antennas" by I. J. Bahi and P. Bhartia, Artech House, for example. With reference to FIGS. 2A and 2B, an exemplary microstrip antenna unit 5 is now described. The microstrip antenna unit 5 comprises a dielectric substrate 6 having a rectangular plane shape. The dielectric substrate 6 is provided on its upper and lower surfaces with a radiating electrode 7 and a shield electrode 8 respectively. The shield electrode 8 is formed substantially over the lower surface of the dielectric substrate 6, excluding a portion to be connected with a coaxial cable and a connector 9. The connector 9 has an inner conductor which is electrically connected to a feeding point 7a of the radiating electrode 7 as shown in FIG. 2B, and an outer conductor which is electrically connected to the shield electrode 8.
The radiating electrode 7 receives/transmits electric waves, so that the microstrip antenna unit 5 operates as an antenna.
When the microstrip antenna unit 5 is miniaturized, however, its gain is disadvantageously reduced. Namely, the gain of the antenna unit 5 is inevitably reduced when the dielectric substrate 6 is reduced in size in order to attain miniaturization. In practice, therefore, the length of the radiating electrode 7, i.e., the size of its longer side cannot be reduced below 1/10 of the wavelength of the waves as transmitted/received, and hence the antenna unit 5 is restricted as to its potential for miniaturization.
Further, the antenna unit 5 cannot be surface-mounted on a printed board or the like since the connector 9 is provided on its bottom surface and projects therefrom. If the connector 9 is removed for enabling surface mounting, it is difficult to attain impedance matching between the antenna unit 5 and a circuit which is connected thereto, and hence its return loss is disadvantageously increased.
FIG. 3 shows an example of the dielectric-loaded monopole antenna unit (c). This monopole antenna unit 11 is fixed to a forward end of a coaxial connector 12. The antenna unit 11 comprises a cylindrical dielectric member 13, and electrode films are formed on an inner peripheral surface of a through hole 13a which is provided in the center of the dielectric member 13 and a forward end surface of the dielectric member 13, to define a radiating electrode. Namely, the dielectric member 13 is arranged around the radiating electrode.
While the antenna unit 11 can be miniaturized due to the aforementioned structure, its gain is still insufficient and the antenna unit 11 cannot be surface-mounted on a printed circuit board since the same is integrated with the coaxial connector 12.